1. Field of the Invention
The present invention relates to an organic EL (electroluminescence) element drive circuit and an organic EL display device using the same drive circuit and, in particular, the present invention relates to an improvement of an organic EL display device in such a way that power consumption of an organic EL display device can be reduced and an organic EL display device having the same organic EL element drive circuit.
2. Description of the Prior Art
Since an organic EL display panel is possible to realize high luminance display by spontaneous light emission, it is suitable for display on a small screen and has been paid an attention as the next generation display device to be mounted on a portable telephone set, a PHS, a DVD player or a PDA (personal digital assistance). The organic EL display device has a problem that, if a voltage drive is employed in the organic EL display device as in a liquid crystal display device, the display control becomes difficult due large variation of luminance and sensitivity difference between R (red), G (green) and B (blue) display colors. In view of this fact, an organic EL display device using a current driver has been proposed.
An organic EL display panel of an organic EL display device of a portable telephone having 396 (132×3) terminal pins for column lines and 162 terminal pins for row lines has been proposed and there is a tendency that the number of column lines and the number of row lines are further increased.
Incidentally, JPH9-232074A discloses a drive circuit for organic EL elements, in which each of the organic EL elements arranged in a matrix is driven by current and a terminal voltage of the organic EL element is reset by grounding an anode and a cathode of the organic EL element. Further, JP2001-143867A discloses a technique with which power consumption of an organic EL display device is reduced by driving organic EL elements by current with using DC-DC converters.
In an organic EL display device, one line on a column side (anode side) becomes a current discharge side and a row side (cathode scan side) becomes a current sink side. A drive current is supplied from a current drive circuit on the column side to the anodes of an organic EL element (referred to as “OEL element”, hereinafter) according to a row side scan. The cathode side of the OEL element is grounded through a CMOS push-pull circuit to sink the drive current. Since the OEL element is a capacitive element, a portion of the drive current is accumulated in the OEL element as electric charge. Therefore, in the display device having a number of OEL elements arranged in matrix, charges of OEL elements surrounding the OEL element flow into the latter OEL element, causing erroneous light emission of the latter OEL element to occur.
In order to prevent such erroneous light emission, a reset period corresponding to a retrace period of a horizontal scan is provided in a display device of such kind to reset anode sides of all OEL elements in one horizontal line, which are connected to respective vertical scan lines in a row side scan (vertical scan), to a constant voltage and cathode sides of the OEL elements of a horizontal line other than those, which are not scanned, are reverse-biased.
According to a recent request of high definition display, there is a tendency that the number of drive pins is increased. Therefore, drive frequency becomes higher and power consumption tends to be increased. When the row side of any other OEL element, which is not scanned, is reverse-biased in order to prevent erroneous light emission, electric charge of the OEL element, an amount of which corresponds to the reverse-biasing is accumulated in a direction opposite to a driving direction of the OEL element. Therefore, when the OEL element is to be scanned, transient current, which is large enough to drive the other OEL elements while canceling out the electric charge accumulated therein, flows. As a result, the increase of drive current by power consumption due to current for accumulating the electric charge corresponding to the reverse-biasing and the transient current become not negligible.